1. Field of the Invention
The invention relates to a composite multilayer material for plain bearings, comprising a backing layer, a bearing metal layer consisting of a copper alloy with a copper content of from 50 to 95 wt. % or an aluminium alloy with an aluminium content of from 60 to 95 wt. %, a diffusion barrier layer and an overlay, applied by electroplating, consisting of a lead-free, tin- and copper-containing alloy.
2. Description of Related Art
Composite multilayer materials with a structure comprising steel backing/lead-bronze/lead-tin-copper overlay have proven very reliable and to have a high mechanical load carrying capacity. Such composite multilayer materials are described in Glyco Ingenieurberichte 1/91, for example.
A typical representative of these groups of materials has the following structure:
steel
CuPb22Sn bearing metal
nickel barrier
PbSn10Cu2 overlay.
The electrodeposited overlay in particular is a multifunctional material, which, inter alia, takes on the following tasks:
embeddability with respect to foreign particles
running-in or conformability of sliding counterparts
corrosion protection for the lead-bronze
emergency running properties in the case of an oil shortage.
The bearing metal likewise contains certain emergency running reserves in case the overlay is completely worn away.
These bearing designs, which have proven effective for decades, today however still contain quantities of lead in the bearing metal and the overlay.
Electrodeposition of the ternary overlay has hitherto mainly been carried out using baths containing fluoroborate. Copper deposition could be carried out only up to a rate of 2 wt. % in these baths, while in cyanide baths deposition rates for the copper of up to 20 wt. % could be achieved. However, it has become clear that the coating is extremely brittle and in this respect is not very durable.
DE-OS 27 22 144 discloses the use, as a soft metal bearing alloy for multilayer plain bearings, of an alloy comprising more than 6 to 10 wt. % copper and 10 to 20 wt. % tin, the rest being lead. This alloy may, inter alia, be applied by electrodeposition, wherein a nickel intermediate layer is provided as a diffusion barrier. However, this known alloy, which is produced using conventional electrolyte baths, exhibits coarse tin distribution.
DE 195 45 427.8 describes fluoroborate-free electroplating baths for the production of overlays of lead-tin-copper, said baths containing a grain-refining agent comprising carboxylic acid and a fatty acid glycol ester. In this way, finely crystalline deposition of the tin is achieved with completely homogeneous distribution thereof, copper deposition of from 2 to 16 wt. % being obtained.
The prior German Application DE 196 22 166 describes ternary layers with hard particles, wherein fluoroborate-free electroplating baths with added alkylsulphonic acid are used for the overlay, to obtain a completely homogeneous distribution of the hard particles in the matrix material. In addition to lead-containing overlays, these alkylsulphonic acid baths may also be used for the application of lead-free layers of SnCuNi, Sn, SnCu and CuSn. However, it has been shown that, although copper contents of up to 16% may be obtained with these lead-free layers, these layers do not exhibit satisfactory properties as far as mechanical load carrying capacity and fatigue strength are concerned.
Investigations using fluoroborate-free baths have shown that it is possible for the overlay to contain up to 30 wt. % copper. Deposition proceeds stably and reliably. Another advantage lies in the fact that no copper is deposited on the steel backing of the bearing.
The hardness of the binary alloy SnCu containing 30 wt. % copper, the rest being tin, is .gtoreq.100 HV. Extended aging at elevated temperature (170.degree. C.-2000 h) indicated a tendency for the overlay elements to diffuse towards the nickel diffusion barrier layer. This may lead to embrittlement and thus to an impairment of the overlay/diffusion barrier layer or bearing metal/diffusion barrier layer bond.
The advantage of the relatively high overlay hardness achievable through the copper content could not therefore hitherto be fully exploited.